A synthetic paper composed of a biaxially stretched polypropylene film containing from 5 to 40% by weight inorganic fine powders as a base material layer in place of a paper made of a natural pulp and a uniaxially stretched polypropylene film containing from 8 to 65% by weight inorganic fine powders laminated onto both surfaces of the base material layer as a paper-like layer is proposed and practically used (see, U.S. Pat. No. 4,318,950, JP-B-46-40794, JP-B-60-36173, JP-B-62-35412, etc., (the term "JP-B" as used herein means an "examined published Japanese patent application")).
Since the synthetic paper with the laminated layer structure has many microvoids around inorganic fine powders as the nuclei and has long cracks at the surfaces thereof, a part of the inorganic fine powders which become the nuclei of the microvoids and surface cracks project from the surface (see FIG. 3), the inorganic fine powders are liable to fall off (so-called paper powder trouble) from the surface layers, the inorganic fine powders that fall off stain a blanket barrel and intermix with a printing ink at offset printing to lower the continuous printing property, and are also the cause of dust in a computer room or a drawing room using CAD, which is undesirable.
Also, when adhesive tape, which is stuck to the printed surface of the synthetic paper, is released, if the surface strength of the paper is weakened, the printing ink is peeled off. Thus, such a synthetic paper is unsuitable as a sheet for a roll-form delayed label (heat-sensitive pressure-sensitive adhesive type label) without need of a released paper.
Furthermore, when using synthetic paper, ink drying at offset printing is not complete and further quick drying has been desired.
For preventing the occurrence of the paper powder trouble, a high glassy synthetic paper composed of a paper-like layer containing inorganic fine powders and a resin film layer containing no inorganic fine powders or containing a very small amount (not more than 3% by weight) of inorganic fine powders, if any, formed on the surface of the paper-like layer as described in U.S. Pat. No. 4,663,216 and a synthetic paper composed of a stretched polyolefin film (a) containing from 10 to 40% by weight inorganic fine powders and a stretched polyolefin film (b) containing no inorganic fine powders laminated on one surface, which becomes a printing surface side, (the thickness of the layer (a)/the thickness of the layer (b) is at least 4) as described in JP-B-3-44909 are proposed.
These synthetic papers are excellent in preventing the occurrence of the paper powder trouble and in surface strength but have the disadvantages that the pencil writing property is poor and drying of an offset printing ink is delayed.
Furthermore, a synthetic paper with a multi-layer resin structure composed of a biaxially stretched thermoplastic resin film as a base material layer (la), having laminated on at least one surface thereof a laminate composed of a uniaxially stretched thermoplastic resin film containing from 8 to 65% by weight inorganic fine powders as a paper-like layer (1b) (in addition, the paper-like layer contains at least 2% by weight inorganic fine powders having a specific area of at least 100 m.sup.2 /g) and a uniaxially stretched styrene-polypropylene copolymer film having a thickness of from 0.15 to 2 .mu.m as a surface layer (1c), wherein the thickness (t) of the surface layer (1c) satisfies the following equation (1) when a mean particle size of the inorganic fine powders existing in the paper-like layer (1b) is defined (R); EQU R.gtoreq.t.gtoreq.1/10.times.R (1)
as described in U.S. Pat. 4,705,719 is proposed.
However, the foregoing synthetic paper is good in the surface strength, the pencil writing property, preventing an occurrence of the paper powder trouble and the gravure printing property but the drying property of the offset printing ink is insufficient.